In heart muscle cells, the contractile units are linked together by Z bands. Despite their vital role in contraction, the nature and organization of the Z bands are unknown. Electron micrographs of human cardiac and skeletal muscle reveal Z bands of various dimensions, but the functional significance of this variety remains obscure. The purpose of these studies is to further determine the organization of Z band components in both normal and widened Z bands and to relate this organization to proposed functions for Z bands. Using optical diffraction methods, we have obtained new information about Z band structure and the relationship of Z components to adjacent thin contractile filaments. From surveys of Z bands in normal, ischemic, anoxic and hypertrophied mammalian hearts and in normal fast and slow skeletal muscle, we have selected papillary muscles and a slow skeletal muscle for further study. Data from these two muscle types have been interpreted on the basis of two, related, three-dimensional models of the Z band. We have found that optical longitudinal sections of cardiac muscle are similar. The widened Z band (l micron) is similar to the normal Z band and is therefore suitable for further analysis by optical filtering and reconstruction techniques. An expanded systematic approach for Z band analysis using goniometer tilt and stereo techniques for electron microscopy at standard and high voltages, together with optical diffraction and reconstruction methods, is proposed. Structural analysis of Z bands in normal, developing and diseased muscle should contribute to our understanding of muscle cell function.